Date: Wed, 25 Sep 2013 20:43:30 +0200
From: Andres Freund <andres@2ndquadrant.com>
To: Jeff Janes <jeff.janes@gmail.com>
Cc: Josh Berkus <josh@agliodbs.com>,
 "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Planner performance extremely affected by an hanging
 transaction (20-30 times)?
Message-ID: <20130925184330.GC5578@awork2.anarazel.de>
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On 2013-09-25 11:17:51 -0700, Jeff Janes wrote:
> On Wed, Sep 25, 2013 at 10:53 AM, Andres Freund <andres@2ndquadrant.com>wrote:
> 
> > On 2013-09-25 00:06:06 -0700, Jeff Janes wrote:
> > > > On 09/20/2013 03:01 PM, Jeff Janes wrote:> 3) Even worse, asking if a
> > > > given transaction has finished yet can be a
> > > > > serious point of system-wide contention, because it takes the
> > > > > ProcArrayLock, once per row which needs to be checked.  So you have
> > 20
> > > > > processes all fighting over the ProcArrayLock, each doing so 1000
> > > > times per
> > > > > query.
> >
> > That should be gone in master, we don't use SnapshotNow anymore which
> > had those TransactionIdIsInProgress() calls you're probably referring
> > to. The lookups discussed in this thread now use the statement's
> > snapshot. And all those have their own copy of the currently running
> > transactions.

> See HeapTupleSatisfiesMVCC, near line 943 of tqual.c:
> 
>         else if (TransactionIdIsInProgress(HeapTupleHeaderGetXmin(tuple)))
>             return false;
>         else if (TransactionIdDidCommit(HeapTupleHeaderGetXmin(tuple)))
>             SetHintBits(tuple, buffer, HEAP_XMIN_COMMITTED,
>                         HeapTupleHeaderGetXmin(tuple));
> 

Hm, sorry, misrembered things a bit there.

> If we guarded that check by moving up line 961 to before 943:
> 
>     if (XidInMVCCSnapshot(HeapTupleHeaderGetXmin(tuple), snapshot))
>         return false;           /* treat as still in progress */
> 
> Then we could avoid the contention, as that check only refers to local
> memory.

That wouldn't be correct afaics - the current TransactionIdIsInProgress
callsite is only called when no HEAP_XMIN_COMMITTED was set. So you
would have to duplicate it.

> As far as I can tell, the only downside of doing that is that, since hint
> bits might be set later, it is possible some dirty pages will get written
> unhinted and then re-dirtied by the hint bit setting, when more aggressive
> setting would have only one combined dirty write instead.  But that seems
> rather hypothetical, and if it really is a problem we should probably
> tackle it directly rather than by barring other optimizations.

I am - as evidenced - too tired to think about this properly, but I
think you might be right here.

Greetings,

Andres Freund

-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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